The present disclosure relates to aluminosilicate glasses. Its main subject matter more specifically concerns aluminosilicate glasses whose compositions may or may not contain boron, and are free of alkali metals.
Said glasses have properties of interest and more particularly a low coefficient of thermal expansion (CTE). With reference to these properties of interest, particular uses of said glasses include substrates for cooktops of induction heating cooking devices. The disclosed glasses have been developed within the context of cooktops for induction heating cooking devices but are in no way limited to the said context.
Within this context, lithium aluminosilicate type glass-ceramic plates are chiefly proposed. Said glass-ceramic plates have been described in numerous documents and in particular in U.S. Pat. No. 7,671,303 and patent application WO 2012/010278.
Three types of such plates in particular are currently marketed: (i) dark plates (black), bulk-colored glass ceramics (of lithium aluminosilicate type) containing a solid solution of β-quartz as a main crystalline phase; (ii) white plates, semi-opaque glass ceramics (of lithium aluminosilicate type) containing a solid solution of β-spodumene as a main crystalline phase; and (iii) transparent plates, colorless glass ceramics (of lithium aluminosilicate type) containing a solid solution of β-quartz as a main crystalline phase having a colored-decorative coating on their lower surface (main lower surface intended to be arranged facing the heating elements).
For this third type of plate, more specifically for the substrate in transparent colorless glass ceramics, there is much room for improvement notably regarding the type of fining agent (of the precursor glass) used to obtain the glass ceramic. Arsenic oxide (As2O3), a toxic product, is still widely used. In lieu and stead of said arsenic oxide, increasing use is made of tin oxide (SnO2), which imparts a yellowish coloring to the glass ceramic. This undesirable yellowish coloring is the result of Ti—Fe, Sn—Fe and Sn—Ti interactions (via charge transfer). Persons skilled in the art well know that the compositions of precursor glasses of glass-ceramics may contain iron and generally TiO2 as a nucleating agent. Associated with such glass-ceramics are elevated costs including manufacturing costs (including price of raw materials and production costs for the manufacturing process including a ceramming step).
The inventors, seeking such improvements took a fully original approach based on the highly advantageous substitution of said prior art plates in glass-ceramic (of lithium aluminosilicate type) by plates in aluminosilicate glass.
The inventors have surprisingly found aluminosilicate glasses whose composition is free of alkali metals, having the required properties for said highly advantageous substitution. It can henceforth be noted here that the substitution is most advantageous insofar as it obviates the implementation of a ceramming step, and therefore the use of nucleating agent(s), the use of lithium oxide (Li2O: a costly raw material) and the use of arsenic oxide (tin oxide being notably fully suitable as a fining agent).
Numerous aluminoborosilicate glasses have been described whose compositions are free of alkali metals, more particularly for application in the technical field of displays (screens, televisions, etc.). Said aluminoborosilicate glasses have especially been described in the following patent documents: U.S. Pat. No. 5,116,788, EP 1 911 725, US 2007/0243992, WO 2004/020356, US 2009/0294773, U.S. Pat. No. 7,727,916, US 2009/0129061, EP 0 787 693, US 2012/0033693 and CN 102515524. In general they have thermal expansion coefficient values higher than 30×10−7/° C. They do not have the composition of the glasses as disclosed herein.